Perform cluster calculation before combining GMM exceedance curves

[pmpowers-usgs]

Currently, exceedance curves for each GMM are combined for each fault in a cluster before computing the conditional probability of exceedance for the cluster. The GMM models are independent logic tree branches and as such should be treated independently when computing hazard for a cluster (compute conditional probabilities of exceedance and then combine). For whatever reason, the suite of GMMs used in 2008 does not exhibit a significant difference between either order of operations, but the 2014 suite yields a higher hazard.

The attached figures show total hazard curves across all source types for Cape Girardeau, MO. The curves have been decomposed and it is the cluster sources that give rise to the discrepancy in each.

[pmpowers-usgs]

The cluster calculation is done correctly in nshnmp-haz and will not be fixed in this codebase.